bug#21242: 24.5.1; python.el: should display output buffer on exceptions when using python-shell-send-region

[Yuri D'Elia <wavexx@thregr.org>]

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When evaluating the code interactively, there's currently no
notification about evaluation errors. Exceptions in the output buffer go
simply unnoticed, unless you watch for them closely. I like to keep the
output buffer buried, and assume that no-notification==no-error.

Exceptions caused by interactive evaluation should either:

- show a notification of sorts in the minibuffer
- display the output buffer for inspection

For inspiration about the second, see:

https://github.com/wavexx/python-x.el/blob/master/python-x.el#L424

Given previous experience with python-mode.el (which tends to break the
current window layout with his own splitting logic), I find it important
that the notification function (which displays the output buffer) should
be customizable, and this is reflected in the example code.

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--- python.el.Orig	2015-07-18 18:00:49.367517569 +0200
+++ python.el	2015-07-18 18:03:01.203659502 +0200
@@ -2548,18 +2548,22 @@
                      (make-string
                       ;; Subtract 2 because of the coding cookie.
                       (- (line-number-at-pos start) 2) ?\n))))
-         (toplevel-block-p (save-excursion
-                             (goto-char start)
-                             (or (zerop (line-number-at-pos start))
-                                 (progn
-                                   (python-util-forward-comment 1)
-                                   (zerop (current-indentation)))))))
+         (block-param (save-excursion
+                        (goto-char start)
+                        (progn
+                          (python-util-forward-comment 1)
+                          (list (current-indentation)
+                                (/= (point) start)))))
+         (block-indentation (car block-param))
+         (starts-with-indentation-p (cadr block-param)))
     (with-temp-buffer
       (python-mode)
       (if fillstr (insert fillstr))
+      (when (and (> block-indentation 0) (not starts-with-indentation-p))
+        (insert (make-string block-indentation ?\s)))
       (insert substring)
       (goto-char (point-min))
-      (when (not toplevel-block-p)
+      (when (> block-indentation 0)
         (insert "if True:")
         (delete-region (point) (line-end-position)))
       (when nomain